Previous studies indicate there are two surges of prolactin during pregnancy in the rat, one occurring during the dark period termed the noctural surge and one occuring in the late afternoon, termed the diurnal surge. These surges continue daily until midpregnancy, at which time they abruptly disappear. Since prolactin is a hormone that plays a critical role during pregnancy, both for maintenance of the corpul luteum and for mammary gland development, as well as regulation of many metabolic functions, it is important to fully understand how its secretion is controlled during this time period. Since the field of neuroendocrinology is developing so rapidly, much information and many techniques can be used to fully explore prolactin control during pregnancy. The specific aims of this study are threefold: (1) To determine what ovarian and brain factors regulate the 2 daily prolactin surges seen during the first 10 days of pregnancy; (2) To determine what causes these surges to disappear at midpregnancy; (3) To determine what regulates their reappearance if the uterus-placentas are removed after midpregnancy. The endogenous factors to be studied include: estradiol and progestrone from the ovaries, thyrotropin releasing hormone, vasoactive intestinal peptide, Beta-endorphin, dopamine and acid extract of the hypothalamus, and rat placental lactogen (rPL). Extensive work will be done investigating the tissue sources of rPL, which might differ since 2 forms of rPL are secreted at each of two different times during pregnancy. It will also be determined whether both the early and late peaks of rPL are effective in inhibition of prolactin surges. Purified rPL will be used to determine the importance of the medial preoptic area, the suprachiasmatic nucleus and the median eminence as sites of feedback on the prolactin surge mechanism. Use will be made of radioummunoassay techniques, stereotaxic surgery, and rPL purification from rat placentas.